An inside CVD method is a method in which glass particles of silica or the like are deposited on the inner wall surface of a starting pipe composed of silica based glass and the deposited glass particles are vitrified to produce an optical fiber preform. The glass particles are synthesized by introducing a gas of a glass raw material, such as silicon tetrachloride (SiCl4), germanium tetrachloride (GeCl4), or the like, into the starting pipe together with an oxygen gas, and then heating the pipe to a high temperature of about 1600° C. so that the raw material gas is subjected to oxidization reaction. In the inside CVD method, a gas burner using a mixed gas of hydrogen gas (H2) and oxygen gas (O2) or a mixed gas of propane (C3H8) and O2 has conventionally been used as a heat source. However, the use of such gas burner has been disadvantageous since hydrogen molecules and hydroxyl groups (OH groups) tend to diffuse into a glass body to be processed, entering from the surface thereof, thereby causing a degradation in the transmission loss of an optical fiber produced from the glass body.
It has been proposed that a plasma burner that does not use hydrogen is adopted as a heat source for heating a glass body. Such a plasma burner is structured such that the cylindrical main body thereof, which is made of silica glass, for example, is inserted into the center of a coil to which a radio-frequency current is supplied. A plasma flame can be generated according to the size of the main body of the burner by introducing argon (Ar) or air thereinto. In Japanese Patent No. 2818735, a method is proposed in which an optical fiber preform for a fiber optic product little containing impurities such as hydrogen molecules and OH groups can be produced using a plasma burner in the inside CVD method. The contamination of a glass body with impurities such as hydrogen molecules, OH groups, and the like, can be significantly decreased by using the plasma burner as compared with the case of using a gas burner.
However, a starting pipe is liable to reach a high temperature in the use of a plasma burner as compared with the case of using a gas burner, since the a plasma fireball has a temperature of several thousands degrees to ten and several thousands degrees When a glass film containing Ge is deposited by the inside CVD method, it is required to maintain the maximum heating temperature low so that the occurrence of bubbles due to the generation of GeO may be suppressed. On the other hand, when a glass film is deposited at a rate of, for example, 1.3 g/min or more from the viewpoint of improvement in productivity, a heating width must be made broader for accomplishing the complete consolidation of the glass film. A drawback of using a conventional plasma burner is that when the deposition of a glass film is conducted at a rate of 1.3 g/min or more, an unconsolidated portion or a bubble tends to occur since widening the heating width of the starting pipe is difficult without increasing the maximum temperature.
A conceivable method for widening the heating width is to increase the nozzle diameter of a plasma burner. For example, as disclosed in Japanese Patent Application Laid open No. H8-195295, the nozzle diameter of a plasma burner can be increased if a conductive material such as carbon is placed inside the burner. However, such a type of plasma burner in which a carbon rod or the like is placed inside is unsuitable as a heat source for producing an optical fiber preform since it has shortcomings such that an optical fiber preform is contaminated with impurities or the heating width changes with an elapse of time, or the like. The heating width can also be widened using an inductive plasma generator disclosed in Japanese Patent Application Laid open No. H7-307199. However, the central temperature of the burner increases since the skin effect decreases as a result of the inductive plasma generator using a low frequency as an ignition source of inductive plasma. Also, the plasma temperature increases since Ar gas is used as a gas to be flowed into the burner. Therefore, the inductive plasma generator is unsuitable as a heat source of the inside CVD method.
Patent Document 1: U.S. Pat. No. 2,818,735
Patent Document 2: Japanese Patent Application Laid open No. H8-195295
Patent Document 3: Japanese Patent Application Laid open No. H7-307199